Fuel cells are composed of individual, serially interconnected cells. These form what is generally referred to as a stack or cell stack, into which additional cooling components are frequently integrated. In this context, a stack may include up to 500 individual cells. A cell is characterized by a layered structure, one cell being composed of an arrangement made up of two bipolar plates having a gas distribution structure, two gas diffusion layers and of a reaction layer in the form of a catalyzed membrane. Two gas diffusion layers surround the reaction layer and form a membrane-electrode assembly. To prevent leakage, the membrane-electrode assembly or the bipolar plate is provided with a seal. Leakage can lead to destruction of the fuel cell, and it degrades the efficiency thereof. From the U.S. Patent Application 2003/0082430 A1, a gas diffusion unit is known where the seal is injection molded onto the planar gas diffusion layer. During assembly, it is difficult to mutually align the gas diffusion layers in a way that imperviously seals the contact regions of the gas diffusion layer and the reaction layer, as well as of the gas diffusion layer and bipolar plate.